Generally, self-contained air conditioning units of the reversible type which are adapted to be mounted in the outer wall of an enclosure and utilized for heating the air from the enclosure during the winter and cooling the air from the enclosure during the summer comprise a housing divided into an indoor section and an outdoor section. An indoor heat exchanger is disposed in the indoor section while an outdoor heat exchanger is disposed in the outdoor section and usually the compressor including the reversing valve are located in the outdoor section.
The compressor is reversibly connected to the heat exchangers through the reversing valve so that the indoor heat exchanger functions as an evaporator when the unit is operating on the cooling cycle and the outdoor heat exchanger functions as the evaporator on the heating cycle. Suitable independent fan means may be provided for circulating indoor air over the indoor heat exchanger and outdoor air over the outdoor heat exchanger during operation of the system on either the heating or cooling cycle.
Under certain operating conditions in the heating cycle, the outdoor heat exchanger functioning as the evaporator may operate at such low outdoor ambient temperatures as to cause the accumulation of a coating or layer of frost on its surface. Since frost when it accumulates operates as a barrier to heat transfer between the evaporator and the air being circulated thereover, the efficiency of the unit is markedly reduced. Further, unless means are provided for interrupting the accumulation of frost, the evaporator can become completely filled with a layer of frost that may effectively block air passage therethrough. This blockage of air results in the loss of heat exchange and if allowed to continue can cause refrigeration system components to fail and can also result in compressor burn-out unless compressor operation is terminated.
The shutting down of compressor operation each time frost accumulates severely curtails the operation of the unit in the heating cycle and accordingly the efficiency of the unit as a heating means at temperatures below the evaporator frosting level.
In some prior art applications such as U.S. Pat. No. 3,159,981--Huskey, assigned to the General Electric Company, assignee of the present invention, a control circuit is utilized to interrupt the operation of the compressor whenever either the outdoor or indoor heat exchanger attains a frosting temperature and further to supply auxiliary heat to the enclosure whenever the operation of the compressor is interrupted during the heating cycle.
In other prior art systems, the refrigeration system is reversed so that the outdoor coil that accumulates frost when it operates as an evaporator functions as a condenser long enough to melt accumulated frost.
In U.S. Pat. No. 3,555,842--Bodcher, a defrost line connects the upper inlet of the condenser to the upper inlet of the evaporator and includes a defrost valve which is closed during operation of the compressor but opens when compressor operation terminates. A return line connects the evaporator collector with the lower part of the condenser and includes a valve which operates in the same manner as the defrost valve.